xfrm: Allow UDP encapsulation only in offload modes

xfrm: Allow UDP encapsulation only in offload modes

The missing check of x->encap caused to the situation where GSO packets
were created with UDP encapsulation.

As a solution return the encap check for non-offloaded SA.

Fixes: 983a73da1f99 ("xfrm: Pass UDP encapsulation in TX packet offload")
Closes: https://lore.kernel.org/all/a650221ae500f0c7cf496c61c96c1b103dcb6f67.camel@redhat.com
Reported-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>